1. Technical Field
This invention relates to solid-state image sensors. In particular, the invention relates to an improved system for arranging microlenses in the radiation receiving apparatus for use with a solid-state image sensor so as to improve the signal to noise ratio of the solid-state image sensor.
2. Background
Solid-state image sensors have broad applications in many areas including commercial, consumer, industrial, medical, defense and scientific fields. Solid-state image sensors convert a received image from an object to a signal indicative of the received image. Solid-state image sensors are fabricated from semiconductor materials (such as silicon or gallium arsenide) and include imaging arrays of light detecting (i.e., photosensitive) elements (also known as photodetectors) interconnected to generate analog signals representative of an image illuminating the device. Examples of solid-state image sensors including charge coupled devices (CCD), photodiode arrays, charge injection devices (CID), hybrid focal plane arrays and complementary metal oxide semiconductor (CMOS) imaging devices.
Solid-state image sensors are fabricated from semiconductor materials (such as silicon or gallium arsenide) and include imaging arrays of light detecting (i.e., photosensitive) elements (also known as photodetectors) interconnected to generate analog signals representative of an image illuminating the device. These imaging arrays are typically formed from rows and columns of photodetectors (such as photodiodes, photoconductors, photocapacitors or photogates), each of which generate photo-charges. The photo-charges are the result of photons striking the surface of the semiconductor material of the photodetector, which generate free charge carriers (electron-hole pairs) in an amount linearly proportional to the incident photon radiation.
Each photodetector in the imaging array receives a portion of the light reflected from the object received at the solid-state image sensor. Each portion is called a picture element or xe2x80x9cpixel.xe2x80x9d Each individual pixel provides an output signal corresponding to the radiation intensity falling upon its detecting area (also known as the photosensitive or detector area) defined by the physical dimensions of the photodetector. The photo-charges from each pixel are converted to a signal (charge signal) or an electrical potential representative of the energy level reflected from a respective portion of the object The resulting signal or potential is read and processed by video processing circuitry to create an electrical representation of the image.
The detecting area of each pixel is typically smaller than the actual physical pixel dimensions because of manufacturing process constraints, the presence of other circuitry in the pixel area (such as the active elements in CMOS imager arrays) in addition to the photodetector and the proximity of adjacent pixels. The percentage ratio of the detector area to the pixel area is typically referred to as the optical xe2x80x9cfill factor.xe2x80x9d
Typically, microlenses (also known as microlenticular arrays or lenslet arrays) increase the effective optical fill factor of a pixel by increasing (i.e., focusing) the amount of radiation that is incident upon the detecting area. The microlens covers an area larger than the detecting area so that the radiation that would normally fall outside the detecting area, is refracted by the microlens to the detecting area of the pixel. Microlenses are typically placed over every pixel in the pixel array to increase the radiation intensity (i.e., increasing the fill factor) that is incident on every pixel.
A problem with this approach is that microlenses placed over every pixel in the pixel array typically required a gap of 0.8 xcexcm between each microlens due to conventional manufacturing requirements. Consequently, for pixels on the order of 3.5xc3x973.5 xcexcm, the microlens has an approximate maximum diameter of 2.7 xcexcm. As a result, a microlens situated above each pixel is only capable of covering about 47% of the pixel area.
An additional problem is that the noise in pixels of different colors is typically similar while; in contrast, the strength of the signals corresponding to the intensity of the incident light on these pixels differs. As an example in a CMOS image sensor, a characteristic of the semiconductor substrate is that the substrate is more sensitive to longer wavelengths of radiation as opposed to the shorter wavelengths. Blue light has a wavelength of 450 xcexcm; green light has a wavelength of 550 xcexcm; and red light has a wavelength of 650 xcexcm. As a result, the pixel is typically 2 to 3 times less sensitive to a given intensity of blue light incident upon the detecting area than it is to the same intensity of incident red light. Thus, in order to provide signals of similar strength, a 2 to 3 times effective gain is presently applied to the blue signal during the post-processing step.
Unfortunately, as a result of having a weaker signal, the signal-to-noise (SNR or S/N) ratio for the blue pixels is lower than for the other pixels. As a result, when a gain is applied to the signal developed by the blue pixel in post-processing, the associated noise of the signal is increased as well. Consequently, the noise from the blue pixel tends to dominate the entire image created by the different signals from the entire pixel array. Thus, the SNR of the entire image (including signals corresponding to red light, green light, and blue light) is limited by the SNR of the blue pixel.
A further problem with this approach is the presence of cross-talk between neighboring pixels because radiation that falls outside the detecting area of the pixels may create electron-hole pairs in the substrate (i.e., generally the semiconductor area outside the photodetector area). The cross-talk is generated when electron-hole pairs, created by light incident upon the semiconductor surface outside the detecting area of a pixel, diffuse into the detecting area of a nearby pixel. These electron-hole pairs typically increase the detected light intensity in the detecting area of a nearby pixel.
Cross-talk significantly effects the signal produced by the blue pixel because of both the weak response of the blue pixel to blue light and the stronger response of the pixels to red and green light. The cross-talk signal in a blue pixel may be close to, if not greater than, the magnitude of the detected signal for blue light. Conventional designs have attempted to solve this problem by using a set of pre-calibrated cross-talk coefficients to subtract the signal due to cross-talk. Unfortunately, a certain amount of color error is caused by part-to-part variations that cannot be corrected.
Accordingly, there is a need to increase the pixel area that may be covered by a microlens and there is a further need to narrow the differential in signal response between different color pixels. There is also additional need to limit the amount of cross-talk between pixels in order to decrease unwanted color error.
A number of technical advances are achieved in the art, by implementation of an arrangement of microlenses in a radiation receiving apparatus of a solid-state image sensor for improving signal to noise ratio. The invention may be broadly conceptualized as a radiation receiving apparatus that comprises a pixel array and one or more microlenses located between the source of radiation and a less sensitive pixel in the pixel array.
For example, this novel arrangement of microlenses in a radiation receiving apparatus may utilize a system architecture that recognizes that within a pixel array, there is typically a less sensitive pixel (i.e. one receiving light in the blue spectrum) and a more sensitive pixel (i.e. one receiving light in the red spectrum). A microlens is placed in physical proximity to the less sensitive pixel in order to decrease the inherent difference in sensitivity between the less sensitive pixel and the more sensitive pixel, in turn increasing the intensity of radiation incident upon the detecting area of the less sensitive pixel. The detecting area may include a photogate or a photodiode for sensing radiation.
In an implementation of this invention, the size of the face area of the microlens in relation to a detecting area in the less sensitive pixel is determined by this inherent difference in sensitivity. The less sensitive pixel converts a given intensity of a first portion of the radiation spectrum into a first signal level. Similarly, the more sensitive pixel will convert a similar intensity of a second portion of the radiation spectrum into a second signal level. Ideally, the ratio of the microlens face area to the detecting area of its associated less sensitive pixel may be substantially similar to a ratio of the second signal level to the first signal level.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.